1. Field of the Invention
The invention relates to an electric power steering system mounted on a vehicle, such as an automobile, and in particular to the function performed when the number of turns of the steering wheel is detected in the system.
2. Description of the Related Art
A steering system for an automobile is configured so that the rotation of the steering shaft and the pinion shaft caused by a turning operation of the steering wheel is converted, through a rack-and-pinion gear, into a linear motion of a steering mechanism that includes a rack shaft, whereby a desired steering angle is given to the steered wheels. In the electric power steering system, the steering assist force corresponding to the steering torque given by the driver is generated by the motor with a speed reducer and applied to the steering mechanism. The steering torque is detected by the torque sensor. The torque sensor detects the amount of torsion of the torsion bar, that is, the difference between the rotation angle on the input side and the rotation angle on the output side when the pinion shaft is turned by the steering shaft through the torsion bar.
Meanwhile, when the absolute steering angle of the steering wheel is detected in addition to the steering torque, it is possible to precisely control the increase and decrease of the steering assist force with the absolute steering angle as well as the steering torque taken into account. The information on the absolute steering angle is required also when systems other than the electric power steering system are controlled, such as when the distribution of the braking force system is controlled in the Antilock Brake System (ABS). Thus, a shaft angle sensor for detecting the rotation angle of the pinion shaft (shaft angle) is provided.
In general, the steering wheel rotates four to five turns between the right and left steering limit positions. The pinion shaft rotates equally. From the viewpoint of sensor mechanism, it is not economically advantageous to use a sensor which can continuously detect the absolute steering angle over the whole rotation range. Thus, in actuality, a sensor, such as a resolver, that detects the angle within a range of 360 degrees is used. The control section that receives the output from such a sensor has to determine the number of turns of the steering wheel at the current position relative to the point of origin. Based on the number of turns and the current output value of the sensor, the absolute steering angle within the whole rotation range is detected. When it comes to what is the point of origin, during straightforward running when the vehicle is running, for example, it may be determined that the steering wheel is at the point of origin. However, when the ignition switch is turned on to start a vehicle from the parked state, neither the point of origin nor the number of turns cannot be determined and the absolute steering angle is uncertain.
Thus, a system, provided with a motor angle sensor for detecting the angular position of the motor in addition to the shaft angle sensor, that detects the absolute steering angle based on the combination of the outputs of these sensors, is available. See Japanese Patent Application Publication No. 2000-296781, for example. This document discloses a system that is configured so that the same combination of the output values of the sensors does not appear twice for the absolute steering angle over the whole rotation range, by setting the ratio of speed reduction by the speed reducer between the pinion shaft and the motor, which are mechanically linked to each other, so that the ratio between the number of turns of the motor and the number of turns of the pinion shaft is a predetermined ratio that is not expressed by an integer number. Such a configuration makes it possible to detect the number of turns and the unique absolute steering angle based on the combination of the output value of the shaft angle sensor and the output value of the motor angle sensor.
In order to detect the absolute steering angle based on the combination of the output value of the shaft angle sensor and the output value of the motor angle sensor as described above, it is necessarily required that the correlation between the outputs of these sensors is unchanged. However, when there are a torsion, a backlash, and expansion and contraction due to temperature change in the mechanical portion that connects between the motor and the pinion shaft, the above correlation slightly varies. As a result, a shift occurs in the combination of the output values of these sensors, so that an erroneous number of turns (typically, the number shifted by one) can be detected and an erroneous absolute steering angle can be detected based on the erroneous number of turns. When this occurs, the control of the steering assist force based on the absolute steering angle cannot be appropriately performed.